Set Socket interface using interface index

[jorritolthuis]

In C, one can set a socket's interface using setsockopt(socket, SOL_IP, IP_MULTICAST_IF, &mreqn, sizeof(mreqn)) where mreqn is

struct ip_mreqn {
    struct in_addr imr_multiaddr; /* IP multicast group address */
    struct in_addr imr_address;   /* IP address of local interface */
    int            imr_ifindex;   /* interface index */
};

sockets2 offers the set_multicast_if_v4 function, but only accepts an address as parameter. I would like to set the interface using sockets2 based on an interface index.

[Thomasdezeeuw]

A pr for this would be welcome. We'll have to look at how the ip_mreqn type would look as it needs to be cross platform(-ish). Another point is the migration of the API as we can't have breaking changing, but in future version we'll want a single function for IP_MULTICAST_IF